Method for operating a pump unit, pump unit and use thereof

ABSTRACT

A method for operating a pump unit including a pump and a drive motor for the latter, the pump unit being disposed in a fluid circuit that is to be filled with a liquid to be moved and in which the liquid to be moved is moved using the pump unit. In order to provide a method in which the pump unit can be operated in a more energy-efficient manner, it is proposed that the fill status of the fluid circuit be determined and, with the use of a control unit, the working point of the pump unit be modified in operation thereof depending on the fill status of the fluid circuit. Also described is a pump unit for carrying out the method and to the use of a pump unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation patent application of international application number PCT/EP2014/071349 filed on Oct. 6, 2014 and claims the benefit of German application No. 10 2013 111 185.8 of Oct. 9, 2013.

The present disclosure relates to the subject matter disclosed in international application number PCT/EP2014/071349 of Oct. 6, 2014 and German application No. 10 2013 111 185.8 of Oct. 9, 2013, which are incorporated herein by reference in their entirety and for all purposes.

FIELD OF THE INVENTION

The present invention relates to a method for operating a pump unit comprising a pump and a drive motor for the pump, said pump unit being disposed in a fluid circuit that is to be filled with a liquid to be moved and in which the liquid to be moved is moved using the pump unit.

Further, the present invention relates to a pump unit for carrying out the method, said pump unit comprising a pump and a drive motor for the latter.

Still further, the present invention relates to a use of a pump unit comprising a pump and a drive motor for the latter.

BACKGROUND OF THE INVENTION

It is known to use a pump unit of the above-mentioned kind in a fluid circuit of what is known as a “draindown” system of a solar thermal collector appliance. To fill the draindown system with liquid to be moved after an interim shutdown to prevent freezing damage, the pump unit pumps the liquid to be moved into the initially empty fluid circuit. Because of the differences in height between the pump unit and the solar thermal collector that occur in practical situations (and which may be for example about 10 m), the pump is required to deliver high power at initially low delivery volume (volumetric flow rate). Typical power input values are of the order of magnitude of about 50 W to 100 W; therefore, the pump unit must be designed for high power. During normal operation of the draindown system, the pump unit circulates the moved liquid through the filled fluid circuit, with the differential pressure between the pump unit's suction and discharge sides being considerably lower. However, the working point the pump unit adopts, as predetermined by the hydraulic characteristic curve adopted by the pump unit, leads to a high volumetric flow rate of moved liquid. The pump unit is oversized for this mode of operation; therefore, energy is unnecessarily consumed when running in the circulation mode of operation.

An object underlying the present invention to provide a method of the generic kind in which the pump unit can be operated in a more energy-efficient manner, as well as a pump unit for carrying out the method.

SUMMARY OF THE INVENTION

In a first aspect of the invention, a method for operating a pump unit comprising a pump and a drive motor for the latter is proposed, said pump unit being disposed in a fluid circuit that is to be filled with a liquid to be moved and in which the liquid to be moved is moved using the pump unit. The method includes determining the fill status of the fluid circuit and, with the use of a control unit, modifying the working point of the pump unit in operation thereof depending on the fill status of the fluid circuit.

In a second aspect of the invention, a pump unit, in particular for carrying out the method, comprises a pump and a drive motor for the latter. The fill status of the fluid circuit in which the pump unit is connected for moving a liquid to be moved is determinable, and the working point of the pump unit is modifiable in operation thereof depending on the fill status of the fluid circuit.

In a third aspect of the invention, a pump unit, in particular for carrying out the method, comprises a pump and a drive motor for the latter. The fill status of the fluid circuit in which the pump unit is connected for moving a liquid to be moved is determinable, and the working point of the pump unit is modifiable in operation thereof depending on the fill status of the fluid circuit. The pump unit is used as a circulation pump unit in a draindown system of a solar thermal collector appliance.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary and the following description may be better understood in conjunction with the drawing figures, of which:

FIG. 1 is a schematic representation of a solar thermal collector appliance comprising a draindown system, with the fluid circuit of the draindown system having connected therein a pump unit constructed in accordance with the invention for carrying out the method in accordance with the invention;

FIG. 2 is a schematic block diagram of the pump unit; and

FIG. 3 is a schematic representation of hydraulic characteristic curves according to which the pump unit can be operated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.

The present invention relates to a method for operating a pump unit comprising a pump and a drive motor for the pump, said pump unit being disposed in a fluid circuit that is to be filled with a liquid to be moved and in which the liquid to be moved is moved using the pump unit. The method includes determining the fill status of the fluid circuit and, with the use of a control unit, modifying the working point of the pump unit in operation thereof depending on the fill status of the fluid circuit.

The method in accordance with the invention provides the possibility of modifying the working point of the pump unit depending on the fill status of the fluid circuit. In particular, this allows the pump unit, which is preferably powered by an inverter, to be operated in accordance with a hydraulic characteristic curve in which the delivery pressure and the volumetric flow rate of moved liquid are adjusted such that filling of the fluid circuit and operation in the circulation mode can be carried out and sustained while keeping the power input of the pump unit as low as possible.

Preferably, a determination is made as to whether the fluid circuit has not yet been filled or whether it has been filled and moved liquid is being circulated. It can be determined whether the pump unit is still performing the fill phase or whether it is performing the circulation phase, and the working point can be modified accordingly.

Preferably, therefore, the working point is modified for minimizing the power input of the pump unit in particular when the fluid circuit is in the filled state. This is in particular possible by the fact that by providing the relatively low differential pressure that is required for running in the circulation mode of operation, the volumetric flow rate is reduced compared to a method of the generic kind, thereby minimizing the amount of energy consumed by the pump unit.

It is advantageous for an internal control unit of the pump unit to be used for determining the fill status of the fluid circuit and for modifying the working point of the pump unit. In this way, the method can have a simple design. In particular, it can be carried out using a pump unit powered by an inverter (inverter pump unit) in which a control unit can control the mode of operation of the drive motor for driving the pump. The need for an external control unit can be eliminated, thereby making the method also cost-effective to implement.

Preferably, internal operating parameters of the pump unit are used for determining the fill status, with it being particularly preferred that only internal operating parameters of the pump unit be used. The need for external sensors which are arranged at the fluid circuit and generate external operating parameters can thereby be eliminated. This makes it possible for the method to be implemented in a cost-effective manner using a design that is simple in structure.

The fill status is preferably determined by determining the differential pressure of moved liquid applied at the pump and/or the volumetric flow rate of moved liquid.

For example, the pump unit can have preset or presettable values of the differential pressure and/or the volumetric flow rate of moved liquid for the circulation mode of operation. This makes it possible, for example by comparing the differential pressure determined and/or the volumetric flow rate determined with one or more preset or presettable values, to determine whether the system is running in the circulation mode of operation and, accordingly, whether the fluid circuit has completely, or substantially completely, filled with liquid to be moved.

To determine the differential pressure and/or the volumetric flow rate of moved liquid, it is advantageous for the power input of the drive motor, the rotational speed of the drive motor and/or characteristic values of hydraulic properties of the pump unit to be used, with said characteristic values being stored in a storage unit. The values of power input as well as the rotational speed of the drive motor are known to the pump unit from the voltage applied at the drive motor and the current at the drive motor. On the basis of these internal operating parameters, a preferably internal control unit of the pump unit is able to determine the differential pressure and/or the volumetric flow rate. In this process, hydraulic properties can be used, in particular the hydraulic characteristic curve of the pump unit. The control unit can read the pump-specific hydraulic properties from a storage unit. For example, in performing the determination, the control unit reads a table stored in the storage unit, said table having stored therein the differential pressure and/or the volumetric flow rate depending on the power input and/or the rotational speed.

It is advantageous for an internal storage unit of the pump unit to be used, whereby the need for an external storage unit can be eliminated. The storage unit is for example integrated in the control unit.

It is advantageous for the modification of the working point of the pump unit to be realized by changing the power input and/or by changing the rotational speed of the drive motor. In an inverter pump unit in particular, this can be accomplished using a design that is simple in structure. The modification of the working point leads to a change in the differential pressure and/or the volumetric flow rate of moved liquid, these values in turn being capable of being determined on the basis of the preferably internal operating parameters of the pump unit. By continuously modifying the working point by changing the power input and/or by changing the rotational speed of the drive motor, it is thereby possible for the hydraulic characteristic curve of the pump unit to be changed in order to adjust a working point for operation, especially for the circulation mode of operation, at as low a power input as possible.

Overall, it is advantageous for the power input and/or the rotational speed of the motor to be controlled depending on a differential pressure determined and/or a volumetric flow rate of moved liquid determined. Operation of the pump unit can thereby be successively controlled to an energy-saving operation in accordance with a hydraulic characteristic curve.

Provision may be made for the working point not to be modified when the differential pressure determined differs from a preset or presettable differential pressure by less than a threshold differential pressure and/or when the volumetric flow rate determined differs from a preset or presettable volumetric flow rate by less than a threshold volumetric flow rate. The preset or presettable values for the differential pressure and/or volumetric flow rate are for example stored in a storage unit of the pump unit and are interpreted by the latter for identifying the circulation mode of operation.

If the differential pressure determined is below a preset or presettable threshold, the control unit can regard this as an indication of reaching the circulation mode of operation if at the same time the volumetric flow rate is above a preset or presettable threshold.

Preferably, the working point of the circulation mode of operation is set by presetting the differential pressure and/or the volumetric flow rate in circulation operation. To this end, the pump unit may comprise a setting element, for example a potentiometer.

Advantageously, the processes of determining the fill status and modifying the working point are performed in real-time. By way of example, the above-mentioned control of the power input and/or the rotational speed takes place in the range of a few milliseconds.

The method is preferably used in filling the fluid circuit of a draindown system with liquid to be moved.

Especially in a draindown system, the fluid circuit need not be closed. The fluid circuit may be what is known as an “open-loop” fluid circuit.

As mentioned at the outset, the present invention further relates to a pump unit for carrying out the method, said pump unit comprising a pump and a drive motor for the latter. The fill status of the fluid circuit in which the pump unit is connected for moving a liquid to be moved is determinable, and the working point of the pump unit is modifiable in operation thereof depending on the fill status of the fluid circuit.

The advantages that can be achieved in conjunction with the method in accordance with the invention can be achieved using the pump unit and in this respect reference is made to explanations given above.

Correspondingly, with respect to the advantageous embodiments of the pump unit constructed in accordance with the invention as mentioned hereinafter, reference may be made to the explanations of advantageous exemplary embodiments of the method in accordance with the invention.

Preferably, a determination is capable of being made as to whether the fluid circuit has not yet been filled or whether it has been filled and the moved liquid is being circulated.

Advantageously, the working point is modifiable for minimizing the power input of the pump unit in particular when the fluid circuit is in the filled state.

Preferably, the pump unit comprises an internal control unit by which, through the pump unit, the fill status of the fluid circuit is determinable and the working point of the pump unit is modifiable.

It is advantageous for internal, and preferably only internal, operating parameters of the pump unit to be usable for determining the fill status.

Advantageously, the fill status is determinable by the differential pressure of moved liquid applied at the pump being determinable and/or by the volumetric flow rate of moved liquid being determinable.

Preferably, for determining the differential pressure and/or the volumetric flow rate of moved liquid, the power input of the drive motor, the rotational speed of the drive motor and/or characteristic values of hydraulic properties of the pump unit are usable, with said characteristic values being stored in a storage unit.

Preferably, the pump unit comprises the storage unit. The storage unit may for example be integrated in a control unit, in particular in a control unit of the pump unit.

It is advantageous for the working point of the pump unit to be variable by changing the power input and/or by changing the rotational speed of the drive motor.

Advantageously, the power input and/or the rotational speed of the drive motor is/are controllable depending on a differential pressure determined and/or a volumetric flow rate of moved liquid determined.

Provision may be made for the working point not to be modified when the differential pressure determined differs from a preset or presettable differential pressure by less than a threshold differential pressure and/or when the volumetric flow rate determined differs from a preset or presettable volumetric flow rate by less than a threshold volumetric flow rate.

Preferably, the pump unit comprises a setting element by which the working point of the circulation mode of operation is adjustable by presetting the differential pressure and/or the volumetric flow rate in circulation operation.

Advantageously, the processes of determining the fill status and modifying the working point are performable in real-time.

As mentioned at the outset, the present invention further relates to a use of a pump unit comprising a pump and a drive motor for the latter. The fill status of the fluid circuit in which the pump unit is connected for moving a liquid to be moved is determinable, and the working point of the pump unit is modifiable in operation thereof depending on the fill status of the fluid circuit. The pump unit is used as a circulation pump unit in a draindown system of a solar thermal collector appliance.

The advantages of the use in accordance with the invention are apparent from the advantages of the method in accordance with the invention and advantageous embodiments thereof as well as of the pump unit constructed in accordance with the invention and advantageous exemplary embodiments thereof; therefore, in this respect, reference may be made to explanations given above.

FIG. 1 is a schematic representation of a solar thermal collector appliance, designated by the reference numeral 10, which comprises what is known as a “draindown” system. The solar thermal collector appliance 10 comprises a collector 12 which is for example mounted on the roof of a house, said collector 12 having connected thereto a feed conduit 14 which on the flow side is connected to a tank 16. The tank 16 stores a liquid to be moved, in particular water. Filling of the tank 16 is through a supply conduit 18 which is in particular connected to a water supply network.

Also connected to the collector 12 is a discharge conduit 20 which opens into the tank 16. On the discharge side, the tank 16 has connected thereto a consumer conduit 22 which feeds to a consumer water that was heated by use of the collector 12.

For moving the water, the feed conduit 14 has connected therein an advantageous embodiment of a pump unit 24 constructed in accordance with the invention which will be explained in more detail below. Downstream of the pump unit 24, the feed conduit 14 and the discharge conduit 20 have connected therein what is called a “draindown” valve 26, of a kind known per se. A drain conduit 28 is connected to the valve 26.

Further connected to the discharge conduit 20, at a position near the collector 12, preferably at the highest point of the solar thermal collector appliance 10, is an aeration and ventilation valve 30.

During normal operation of the solar thermal collector appliance 10, an open fluid circuit 32 (“open loop”) is formed that is filled with water which is supplied through the supply conduit 18 and is discharged through the consumer conduit 22. Circulation of the water is accomplished by use of the pump unit 24.

For freeze protection of the solar thermal collector appliance 10 it is known to interrupt the fluid communication of the feed conduit 14 and the discharge conduit 20, thereby isolating the collector 12 from the tank 16. Drainage of the fluid circuit 32 on the collector 12 side is accomplished through the drain conduit 28.

When the solar thermal collector appliance 10 is returned to service, the fluid circuit 32 has to be filled with water. To this end, the pump unit 24 pumps water from the tank 16 through the feed conduit 14 and into the collector 12, which is arranged at the highest point of the fluid circuit 32.

Because of the difference in height between the pump unit 24 and the collector 12 (which may be for example about 10 meters), the pump unit 24 has to supply a relatively large amount of initial pressure. The initial volumetric flow rate is small compared thereto. On the basis of the volumetric flow rate-pressure diagram for the pump unit 24 depicted in FIG. 3, there is schematically illustrated a working point A where the pump unit 24 has pumped water through the feed conduit 14 into the collector 12 up to the highest head (pressure p_(A), volumetric flow rate V_(A)). Thereafter, the fluid circuit 32 starts to fill in the section of the discharge conduit 20.

To allow initial filling of the fluid circuit 32 based on the difference in height between the collector 12 and the pump unit 24, the latter must be designed for a relatively high pumping power, for example approximately 50 W to 100 W. As the fill level of the fluid circuit 32 increases, the pressure that is required for continued filling and that is to be supplied by the pump unit 24 decreases because water flowing from the collector 12 through the discharge conduit 20 and into the tank 16 causes a reduction in the differential pressure between the suction and discharge sides of the pump unit 24. In this condition, no hydrostatic pressure need be overcome any longer, but instead only the flow losses need be compensated. Therefore, with the fluid circuit 32 completely filled, circulation can be maintained at a circulation pressure p_(E) that is smaller when compared to the maximum pressure p_(A).

If, as is conventional, the pump unit 24 is operated in accordance with a predetermined hydraulic characteristic curve 34, then at a point in time when the fluid circuit 32 is filled and end pressure p_(E) is reached, this leads to a volumetric flow rate V_(E) at end point E (FIG. 3). This proves disadvantageous because the volumetric flow rate V_(E) associated with the end pressure p_(E) would not have had to be supplied per se, meaning that the pump unit 24 is oversized for the circulation mode of operation. Accordingly, when running in circulation mode at working point E, the pump unit 24 has a high power input that is not necessary per se.

For operation in the circulation mode, it would be sufficient for example to use a pump unit (not shown) that is capable of being operated in accordance with a further characteristic curve 36 so that a circulation pressure p_(Z) at a reduced volumetric flow rate V_(Z) can be provided at a circulation point Z. The volumetric flow rate V_(Z) is considerably smaller when compared to the volumetric flow rate V_(E); therefore, the pump unit has a considerably smaller power input of the order of magnitude of a few watts, for example about 5 W to 10 W. However, the further pump unit that is operated in accordance with the characteristic curve 36 cannot provide the pressure p_(A) needed to reach the maximum head.

With the assumption of a square resistance characteristic curve 37 of the fluid circuit 32, the circulation pressure p_(Z) at working point Z is below the circulation pressure p_(E) at working point E. The working point Z is where the resistance characteristic curve 37 intersects the characteristic curve 36.

For avoiding the use of two pump units and for saving energy, a method in accordance with the invention is possible with the pump unit 24 constructed in accordance with the invention and with the use thereof in accordance with the invention. The possibility exists for determining the fill status of the fluid circuit 32 and for modifying the working point of the pump unit 24 depending on the fill status such that as a result, the power input of the pump unit 24 is minimized and operation of the pump unit 24 at circulation point Z, at low circulation pressure p_(Z) and low volumetric flow rate V_(Z), is possible.

Accordingly, a determination can be made as to whether the fill phase or the circulation phase is being performed and depending on such determination the working point of the pump unit 24 can be modified for minimizing the power thereof.

It is in particular possible to determine the fill status and modify the working point by only using the pump unit 24 itself and changing it in its mode of operation.

To this end, the pump unit 24 comprises a pump 38 connected in the feed conduit 14 and a drive motor 40 therefor. Furthermore, the pump unit 24 has a control unit 42 in bidirectional communication with the drive motor 40. The control unit 42 can be used to adjust the operating mode of the drive motor 40, thereby influencing the operating mode of the pump 38. Conversely, the control unit 42 can be used to read and evaluate operational parameters of the drive motor 40.

The pump unit 24 further comprises a storage unit 44 which in the present case is preferably integrated in the control unit 42.

In operation of the pump unit 24, when filling the fluid circuit 32, the control unit 42 can determine the power input and the rotational speed of the drive motor 40. On the basis of the power input and the rotational speed, the control unit 42 is able to determine the differential pressure applied at the pump 38 and the volumetric flow rate of water delivered. In this process, the control unit 42 can use pump-specific characteristics, stored in the storage unit 44, for determining the differential pressure and the volumetric flow rate, with said pump-specific characteristics being given for example by the construction of the pump.

The above-mentioned determination process enables the control unit 42 to determine what the working point of the pump unit 24 is on the volumetric flow rate-pressure diagram in accordance with FIG. 3. Said working point varies as the fill level of the fluid circuit 32 increases, whereby the control unit 42 can, on the basis of the values determined for the differential pressure and volumetric flow rate, make an assessment as to whether the fluid circuit 32 is filled. It is in particular possible to make an assessment as to whether the fill phase is still present or whether the circulation phase is already present.

There is the possibility of controlling the working point of the pump unit 24 to a hydraulic characteristic curve 46 in order to go from working point A, where the maximum head is reached, to circulation point Z, where the pump unit 24 can be operated at circulation pressure p_(Z) and reduced volumetric flow rate V_(Z), thus saving energy.

To this end, the working point of the pump unit 24 can be modified by the control unit 42 by changing, and in particular reducing, the power input and the rotational speed of the drive motor 40 depending on the values determined for the differential pressure and the volumetric flow rate. On the basis of the changed operating parameters of power input and rotational speed, the control unit 42 can then again and repeatedly determine the changed differential pressure and the changed volumetric flow rate in order for the pump unit to be operated in accordance with the characteristic curve 46.

As a result, this allows the power input and the rotational speed of the motor to be controlled depending on the differential pressure and volumetric flow rate determined. The working point of the pump unit 24 can thereby be modified based solely on internal operating parameters, using the control unit 42, depending on the fill status of the fluid circuit 32. In particular, there is no need to make available sensors external to the pump unit 24 for providing operating parameters and/or to make available an external control unit.

The pump unit 24 can store therein preset information related to the circulation pressure p_(Z) and the volumetric flow rate V_(Z) at circulation point Z, to which circulation pressure and/or volumetric flow rate the pump unit 24 is controlled.

In particular when it is not known in what kind of solar thermal collector appliance the pump unit is to be used, provision may be made for the circulation pressure p_(Z) or the volumetric flow rate V_(Z) to be presettable. By way of example, a setting element 48, for example a potentiometer, is provided which can be used by a user to set or preset the volumetric flow rate V_(Z) or the circulation pressure p_(Z) at circulation point Z. It is thereby possible for the pump unit 24 to be adapted to different solar thermal collector appliances, these differing for example in the number of collectors 12 they have.

If the differential pressure determined is below the presettable differential pressure p_(Z), the control unit 42 can regard this as an indication of reaching the circulation mode of operation if at the same time the volumetric flow rate is above a presettable threshold.

In performing the control procedure, the control unit 42 can make a determination as to whether the differential pressure determined and the volumetric flow rate determined differ from the circulation pressure p_(Z) and from the reduced volumetric flow rate V_(Z) by less than a preset or presettable threshold. If this is the case, then the circulation point Z can be maintained without changing the power input and the rotational speed of the drive motor 40.

Controlling the mode of functioning of the pump unit 24 to the hydraulic characteristic curve 46 can essentially be accomplished in real-time. By way of example, the control unit 42 determines the power input and the rotational speed of the drive motor 40 at short intervals, for example at intervals of a few milliseconds. Correspondingly, the power input and the rotational speed of the drive motor 40 can be adjusted at short intervals for modifying the working point.

At circulation point Z, the power input of the pump unit 24 is minimized. It is therefore possible, by use of the pump unit 24 and by carrying out the method, for the solar thermal collector appliance 10 to be operated in an energy-efficient circulation mode of operation. 

What is claimed is:
 1. A method for operating a pump unit comprising a pump and a drive motor for the pump, the method comprising: moving liquid in a fluid circuit using the pump unit, determining a fill status of the fluid circuit, and modifying a working point of the pump unit during operation of the pump unit with a control unit depending on the fill status of the fluid circuit.
 2. The method in accordance with claim 1, further comprising determining whether the fluid circuit has not yet been filled or whether the fluid circuit has been filled and moved liquid is circulating.
 3. The method in accordance with claim 1, wherein the working point is modified for minimizing a power input of the pump unit when the fluid circuit is in a filled state.
 4. The method in accordance with claim 1, wherein an internal control unit of the pump unit is used for determining the fill status of the fluid circuit and for modifying the working point of the pump unit.
 5. The method in accordance with claim 1, wherein internal operating parameters of the pump unit are used for determining the fill status.
 6. The method in accordance with claim 1, wherein the fill status is determined by determining at least one of a differential pressure of moved liquid applied at the pump and a volumetric flow rate of moved liquid.
 7. The method in accordance with claim 6, wherein for determining the differential pressure and/or the volumetric flow rate of moved liquid, at least one of a power input of the drive motor, a rotational speed of the drive motor and characteristic values of hydraulic properties of the pump unit are used, with said characteristic values being stored in a storage unit.
 8. The method in accordance with claim 1, wherein the working point of the pump unit is modified by changing at least one of a power input and a rotational speed of the drive motor.
 9. The method in accordance with claim 8, wherein the power input and/or the rotational speed of the motor are or is controlled depending on at least one of a differential pressure and a volumetric flow rate of moved liquid determined.
 10. The method in accordance with claim 9, wherein the working point is not modified when the differential pressure determined differs from a preset or presettable differential pressure by less than a threshold differential pressure and/or when the volumetric flow rate determined differs from a preset or presettable volumetric flow rate by less than a threshold volumetric flow rate.
 11. The method in accordance with claim 1, wherein the working point of a circulation mode of operation is set by presetting at least one of a differential pressure and a volumetric flow rate in circulation operation.
 12. The method in accordance with claim 1, wherein the determination of the fill status and the modification of the working point are performed in real-time.
 13. The method in accordance with claim 1, wherein the method is used in filling fluid circuit of a draindown system of a solar thermal collector appliance with fluid.
 14. A pump unit comprising: a pump, a drive motor for the pump, and an internal control unit configured to (i) determine a fill status of a fluid circuit in which the pump unit is connected for moving a liquid to be moved, and (ii) modify a working point of the pump unit during operation thereof depending on the fill status of the fluid circuit.
 15. The pump unit in accordance with claim 14, the internal control unit is configured to determine whether the fluid circuit has not yet been filled or whether it has been filled and moved liquid is circulating.
 16. The pump unit in accordance with claim 14, wherein the working point is modifiable for minimizing a power input of the pump unit when the fluid circuit is in a filled state.
 17. The pump unit in accordance with claim 14, wherein internal operating parameters of the pump unit are usable for determining the fill status.
 18. The pump unit in accordance with claim 14, wherein the fill status is determinable by at least one of a differential pressure of moved liquid applied at the pump and a volumetric flow rate of the moved liquid.
 19. The pump unit in accordance with claim 18, wherein, for determining the differential pressure and/or the volumetric flow rate of moved liquid, at least one of a power input of the drive motor, a rotational speed of the drive motor and characteristic values of hydraulic properties of the pump unit are usable, with said characteristic values being stored in a storage unit.
 20. The pump unit in accordance with claim 14, wherein the working point of the pump unit is variable by changing at least one of a power input and a rotational speed of the drive motor.
 21. The pump unit in accordance with claim 20, wherein the power input and/or the rotational speed of the drive motor are or is controlled depending on at least one of a differential pressure and a volumetric flow rate of moved liquid.
 22. The pump unit in accordance with claim 21, wherein the working point is not modified when a differential pressure differs from a preset or presettable differential pressure by less than a threshold differential pressure and/or when a volumetric flow rate determined differs from a preset or presettable volumetric flow rate by less than a threshold volumetric flow rate.
 23. The pump unit in accordance with claim 14, wherein the pump unit comprises a setting element by which the working point of a circulation mode of operation is adjustable by presetting at least one of a differential pressure and a volumetric flow rate in circulation operation.
 24. The pump unit in accordance with claim 14, wherein the determination of the fill status and the modification of the working point are performable in real-time.
 25. A method for using of a pump unit comprising a pump and a drive motor for the pump, the method comprising: determining a fill status of a fluid circuit in which the pump unit is connected for moving a liquid to be moved, modifying a working point of the pump unit during operation of the pump unit depending on the fill status of the fluid circuit, and using the pump unit as a circulation pump unit in a draindown system of a solar thermal collector appliance. 